1. Field of the Invention
The present invention relates to a thin film transistor and a display device or a liquid crystal display device having the same. More particularly, the present invention relates to the improvement of the characteristics of a thin film transistor and the improvement of the display quality of a display device through suppression of leakage current.
2. Description of the Related Art
In a display device such as a liquid crystal display device, display control of respective pixels is performed using thin film transistors. Generally, in a thin film transistor, a gate electrode is arranged to be opposed to a semiconductor film, and an area of the semiconductor film that is opposed to the gate electrode becomes a channel area. At both ends of the channel area, a source electrode and a drain electrode are arranged, respectively. A thin film transistor having a reversely staggered structure (etched channel structure), in which the gate electrode is more closely positioned on a light source side than the semiconductor film, is preferable in terms of production cost. This is because when light from a light source such as a backlight or the like is irradiated onto the thin film transistor having the above-described structure, the gate electrode itself functions as a shielding mask against the opposed semiconductor film.
If the semiconductor film is irradiated with light, hole-electron pairs may occur. Further, due to the hole-electron pairs, leakage current may flow. Accordingly, in terms of occurrence of the hole-electron pairs, it is preferable that the gate electrode film that functions as a shielding mask has a large area and is opposed to the semiconductor film.
In the case where a voltage is applied between a gate electrode and a source electrode (drain electrode), a strong electric field occurs near the boundary of the source electrode (drain electrode) and the semiconductor film. Due to this strong electric field, the leakage current is increased. Accordingly, in terms of occurrence of the strong electric field, if the gate electrode is opposed to the source electrode (drain electrode) in the stacking direction, it is preferable that the opposed area be small, and it is more preferable that the gate electrode be not opposed to the source electrode (drain electrode).
FIG. 9A is a conceptual view illustrating the structure of a thin film transistor in the related art. A semiconductor film SCF is arranged above a gate electrode film GF, and a source electrode film SF and a drain electrode film DF are respectively arranged above both ends of the semiconductor film SCF through an impurity semiconductor film IDS. Here, parts of the gate electrode film GF, the source electrode film SF, and the drain electrode film DF function as a gate electrode, a source electrode, and a drain electrode, respectively. Further, light from a light source is irradiated from the lower side to the upper side in the drawing. The light from the light source is indicated by arrows in the drawing.
In the thin film transistor illustrated in FIG. 9A, as seen from the stacking direction (upward/downward direction in the drawing), the gate electrode film GF does not have an area that overlaps the source electrode film SF and the drain electrode film DF. That is, the gate electrode film GF is offset from the source electrode film SF and the drain electrode film DF. As a result, even in the case where a voltage is applied between the gate electrode film GF and the source electrode film SF (drain electrode film DF), a strong electric field does not occur near the boundary of the source electrode film SF (drain electrode film DF) and the semiconductor film SCF in comparison to a case where the gate electrode film GF overlaps the source electrode film SF (drain electrode film DF).
However, as seen from the irradiation direction (upward/downward direction in the drawing) of the light from the light source, the gate electrode film GF overlaps only a part of the semiconductor film SCF. That is, the gate electrode film GF is opposed to only a part of the semiconductor film SCF. Accordingly, the gate electrode film GF does not sufficiently function as a shielding mask with respect to the semiconductor film SCF. That is, the thin film transistor illustrated in FIG. 9A has a structure in which hole-electron pairs are liable to occur in the semiconductor film SCF due to the light irradiated from the light source. As a result, leakage current occurs. In the drawing, the occurring hole-electron pairs are schematically illustrated, and if a voltage is applied between the gate electrode film GF and the source electrode film SF, holes or electrons of the occurring hole-electron pairs reach the source electrode film SF to cause the leakage current to occur.
FIG. 9B is a schematic view illustrating the structure of a thin film transistor in the related art. In the same manner as the thin film transistor illustrated in FIG. 9A, the gate electrode film GF overlaps the semiconductor film SCF as seen from the light irradiation direction. That is, the gate electrode film GF is opposed to the semiconductor film SCF. However, in comparison to the thin film transistor illustrated in FIG. 9A, the gate electrode film GF is widened to the left and right sides in the drawing, and thus an area where the gate electrode film GF and the semiconductor film SCF overlap each other becomes larger. Because of this, the gate electrode film GF functions more as a shielding mask with respect to the semiconductor film SCF. That is, in the thin film transistor illustrated in FIG. 9B, the gate electrode film GF prevents the light from the light source from being irradiated onto the semiconductor film SCF as the shielding mask, and thus it is difficult for the hole-electron pairs to occur in the structure of the thin film transistor.
However, as seen from the stacking direction, the gate electrode film GF also overlaps the source electrode film SF and the drain electrode film DF. That is, the gate electrode film GF is opposed to the source electrode film SF and the drain electrode film DF. As a result, if a voltage is applied between the gate electrode film GF and the source electrode film SF (drain electrode film DF), a strong electric field occurs near the boundary of the source electrode film SF (drain electrode film DF) and the semiconductor film SCF. Due to this strong electric field, the leakage current is increased. In the drawing, areas where the strong electric field is liable to occur are indicated by surrounding dashed lines.
FIG. 9C is a conceptual view illustrating the structure of a thin film transistor in the related art. In the same manner as the thin film transistors illustrated in FIGS. 9A and 9B, the gate electrode film GF is opposed to the semiconductor film SCF, and the widened state of the gate electrode film GF is between the thin film transistor illustrated in FIG. 9A and the thin film transistor illustrated in FIG. 9B. That is, in comparison to the gate electrode film GF illustrated in FIG. 9A, the gate electrode film GF functions more as the shielding mask, but in comparison to the gate electrode film GF illustrated in FIG. 9B, it functions less as the shielding mask. Further, as seen from the stacking direction, the gate electrode film GF is opposed to parts of the source electrode film SF and the drain electrode film DF, respectively.
In the thin film transistor illustrated in FIG. 9C, the hole-electron pairs occur due to the light irradiated from the light source, and this causes the leakage current to occur. Further, since the gate electrode film GF is also opposed to a part of the source electrode film SF (drain electrode film DF), the leakage current due to the occurring hole-electron pairs becomes much greater by the strong electric field that occurs near the boundary of the corresponding part of the source electrode film SF (drain electrode film DF) and the semiconductor film SCF. In the drawing, areas where the strong electric field is liable to occur are indicated by surrounding dashed lines, and the occurring hole-electron pairs are schematically illustrated.
In the thin film transistors in the related art, even if the widening state of the gate electrode film GF is adjusted, it is difficult to suppress both the occurrence of the hole-electron pairs and the occurrence of the strong electric field.